In the single cell field, I feel like the focus is now more on modalities integration to analyze datasets throughout different angles, either analyzing them separately and make a story out of it, or try to use neural networks to make connection between datasets.
A massive amount of papers are coming out with various machine learning models to do modality predictions or detection of potential hidden features like enhancers and transcription factors. In my opinion, they are at the moment hard to rely on because those methods are too specifically trained and will fail if transpose to an alternative context. This area is growing so fast that there is no time to test new models to give feedback to improve them. Every week a new method is out and no validated and standardized methods stand out.
The spatial aspect is taking over single cell little by little. It is still a challenge to get spatial resolution and single cell resolution at the same time, but we are slowly going there. Genes panels for trancriptomic datasets can be used to get both resolutions but I have no doubt it will soon be possible to get chromatin accessibility, histones marks, proteomic... both spatially and at single cell resolution.
Labs are trying DIY solutions to get single cell resolution in-house to not rely on 10Xgenomics too much, with for example Smartseq3.
As GenoMax mentioned, the number of single cell datasets and atlases is massive. One cornerstone would be to homogenize those datasets to have a curated database of cell type, disease contexts and development.
In the next few years, I think the association of perturbation screening (by activation, inactivation or know out) and single cell, will be a major focus to decipher the mechanistic of genes pathways regulation. From my experience, the bottleneck is to get the sweet spot of infection rate to have an homogeneous and sufficient number of cells infected by each single guide.
Last but not least, when single cell arose, post transcriptional events where not the major focus and everyone jumped on gene expression. Now that the hype is going down, looking at alternative splicing events is slowly coming back, at single cell resolution this time.
On a futuristic note, the number of single cell datasets is so gigantic that it would be possible to infer one modality using another via specific neural networks as autoencoders.